class A {
  f1() {
    console.log("f1: ", this.v);
  }
}
class B {
  constructor() {
    this.v = 10;
  }
  f2() {
    console.log("f2!!!");
  }
}
// B.prototype.f3 = function () {
//   console.log("f3!!!");
// };
// console.log(Object.keys(A));
B.prototype.f1 = A.prototype.f1;

const b = new B();
b.f1();
